1. Field of the Invention
This invention relates generally to methods and apparatus for measurement and control of temperature and more particularly for measurement and control of a substrate temperature within a semiconductor process chamber.
2. Description of the Related Art
Photoresist layers are typically applied and patterned over surfaces of a substrate prior to the formation of features during the manufacture of semiconductor devices. Upon completion of these processes the patterned photoresist must be removed through photoresist stripping or ashing. Quite often a photoresist asher removes a photoresist layer by reacting free radical oxygen atoms with the resist material at an elevated temperature. In addition, the photoresist asher often incorporates a microwave or radio frequency (RF) plasma generator to produce free radical oxygen atoms and oxygen ions, which in turn strip the photoresist through high temperature oxidation. The temperature of the substrate directly impacts the rate of removal of the resist during the process. Photoresist stripping is employed primarily after implant operations where selected areas of a substrate are implanted or xe2x80x9cdopedxe2x80x9d by elements such as boron and phosphorous to define the transistors in an integrated circuit. In another application, patterned photoresist defines regions of dielectric or metal layers that must be removed to form the interconnect wiring that constitute an integrated circuit (IC).
The introduction of new materials and the shrinking of feature sizes in integrated circuits are requiring the development of new photoresist formulations and processes. Tight temperature control of the substrate in an ashing chamber is critical as devices on the substrate can become damaged if the temperature overshoots a set point temperature. On the other hand, the process may not run efficiently if the temperature is not high enough. For example, photoresist stripping rate has been measured to increase by approximately 300 xc3x85 per minute for each 1xc2x0 C. change in substrate temperature. One skilled in the art would appreciate that an inaccurate temperature measurement and control can have disastrous consequences for overall device yield. In ashing chambers where the substrate is being heated by lamps, closed loop temperature control is applied to accurately maintain a temperature set point. In a closed loop temperature control system, the temperature of an object, such as the substrate, is measured and the feedback from the temperature measurement is used by a power control system that controls the intensity of a heating source to increase or decrease the temperature.
FIG. 1A displays a block diagram 100 representing a prior art closed loop controller for the temperature of a substrate in an ashing chamber. In diagram 100, a sensor 104 measures the temperature of a substrate 102. A signal corresponding to the temperature measured by the sensor is sent to the controller, which in turn controls the intensity of heat lamps 108 according to the difference in values of the temperature of the substrate and a set point temperature.
The sensor 104 of diagram 100 is a critical component of the control loop since it needs to provide accurate temperature measurement and must be capable of withstanding the harsh environment of the ashing chamber. FIG. 1B illustrates a detailed view of one prior art sensor employed to measure substrate temperature, where lamps are used to heat the substrate. This sensor consists of a thermocouple bead 111 contacting the substrate through an aluminum pad 103 where the aluminum pad 103 and thermocouple is attached to a pin supporting the substrate 102. FIG. 1B illustrates a diagram of the contact between the substrate 102 and a thermocouple sensor pad 103. Since the backside surface of the substrate and the aluminum pad surface are not completely smooth, a gas gap 110 exists at the interface of the two surfaces. The gas gap 110 exists even though substrate support extension 101 provides a gimballing effect to support pad 103 against the backside surface of the substrate since the corresponding surfaces are not completely smooth. The gas gap 110 causes inaccuracies in temperature measurement especially under the operating condition for ashing processes as is explained further below. The probe body 105 encases thermocouple wires 109a and 109b, which are routed through a high vacuum seal since the chambers typically operate under low pressures below 2 torr.
As mentioned above, the ashing process is performed at a low process pressure, typically below 2 Torr. Therefore, very little gas exists in the gas gap 110 to conduct the heat from the substrate to the contact pad. As a result of the gas being evacuated from the gas gap 110, the effective thermal conductivity is low, which in turn makes it difficult to accurately measure the temperature of the substrate. This method of temperature measurement requires detailed calibration of each individual sensor. Such calibration is normally performed using instrumented substrates. Consequently, the accuracy of such calibration is dependent on the reproducibility of the quality of the substrate-pad contact. Additionally, the characteristics of thermal interaction between substrate and pad vary with both pressure and substrate temperature. This necessitates detailed calibration over an extensive range of temperatures and operating pressures. Furthermore, the contact between the substrate and the aluminum pad is different for each substrate, thus injecting additional variables into the temperature measurement process, not to mention the poor calibration resulting from the substrate-to-substrate inconsistencies. The aluminum pad also oxidizes over time, thereby changing the characteristics of the pad for each process, which in turn further throws off the calibration.
Another type of sensor used in ashing chambers is an optical emissivity sensor. Processing chambers, where the substrate is supported by an RF-excited platen to generate the plasma, cannot use thermocouples since the wires of the thermocouple act as antennas. Because the high voltages induced in the thermocouple lead wires can damage sensitive electronic circuitry to which the thermocouple wires are connected, unshielded thermocouples are typically not used in the presence of RF-excited platens. Therefore, an optical emissivity sensor may be used to measure the temperature of the substrate to avoid this antenna effect. FIG. 1C illustrates block diagram 112 representing a prior art processing chamber employing an optical emissivity probe 124 to measure the temperature of the substrate 102. The chamber 126 includes a microwave source 114 and a radio frequency (RF) source 118. When the chamber 126 is operating in the microwave mode, i.e. microwave source activated and RF source deactivated, pins 128 lift the substrate 102 off of the platen 120. In the RF mode the substrate 102 rests on the platen 120. Here, the sensor assembly, including the contact pad 122, is used as one support in conjunction with the pin 128 when the substrate is elevated for microwave processing. Typically, microwave processing is performed at elevated temperatures as high as 300xc2x0 C. for which lamps 108 are employed.
The optical sensor 124 of FIG. 1C measures temperature of the substrate 102 by detecting the emitted infrared radiation from the backside of the pad 122 which is in contact with the substrate 102. Similar problems as encountered with thermocouples persist with the optical sensor 124. The pad 122 for the optical sensor also oxidizes over time. Accordingly, the emissivity of the backside of the pad changes with time. In addition, the optical sensor 124 must be contained in a light-proof housing 123. Since the lamps 108 emit high intensity radiation, any light leak through the housing 123 of the optical sensor 124, could trigger the optical sensor to measure temperature of the lamps 108, which is much higher than that of the substrate. Furthermore, the optical sensor 124 must be thermally isolated from the chamber body to prevent inaccurate measurement of substrate temperature due to local cooling of the substrate, since the sensor is being heated at one end and cooled at the other end.
For this reason, pyrometers that directly measure substrate temperature are used in most Rapid Thermal Processing (RTP) chambers where substrate temperature is typically above 600xc2x0 C.. At 600xc2x0 C. a semiconductor substrate is opaque to the incident radiation from the lamps, and hence blocks incident radiation of the lamps from the pyrometer. However, at temperatures below 600xc2x0 C., the substrate is not opaque to the incident radiation from the heating lamps. Therefore, radiation from the lamps will be incident on the pyrometer through the substantially transparent substrate. The pyrometer will thus be reading the lamps"" filament temperature rather than the substrate temperature. In essence, the transmittance (xcfx84), which is defined as the fraction of radiant energy that is transmitted through the substrate, is approximately 0 at temperatures greater than 600xc2x0 C. As the temperature of the substrate decreases below 300xc2x0 C., the transmittance of the substrate substantially increases. Below 300xc2x0 C., which is a typical temperature range for ashing and photoresist stripping processes, the substrate transmittance increases to 0.8 for the range of wavelengths emitted by a heat source, such as a halogen lamp used in an ashing chamber. Accordingly, pyrometers designed for RTP processes are not useful at the temperatures encountered in an ashing chamber or in photoresist stripping processes.
In spite of these properties, previous practitioners of pyrometry for RTP processes have had to implement various sophisticated correction algorithms. Some practitioners have had to implement additional probes in an RTP chamber to compensate for the effect of background radiation on substrate temperature measurement. Others have had to provide a reference source of radiation, separate from that emitted by the heat source, to measure reflective and absorptive response of the substrate to this reference source, and to then infer substrate temperature from such response. In summary, current state-of-the-art does not provide for dynamic substrate temperature measurement with reasonable accuracy at temperatures normally used for photoresist stripping. Moreover, current methods in use for resist stripping chambers employ contact pads that require extensive and frequent calibration to minimize any drift due to pad degradation. Also, pyrometry that is typical in high temperature RTP processes is not feasible in photoresist stripping because of the high transmissivity and low emissivity characteristic of semiconductor substrates at resist stripping temperatures.
As a result, there is an urgent need to solve the problems of the prior art to provide a non-contact temperature measuring device capable of accurately operating at low temperatures that are typical in certain semiconductor processes such as photoresist stripping.
Broadly speaking, the present invention fills these needs by providing a method and apparatus for measuring the temperature of a body from a remote location. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, or a device. Several inventive embodiments of the present invention are described below.
In one embodiment, an apparatus for measuring and maintaining a substantially constant temperature of a substrate in a processing chamber is provided. The processing chamber includes a heating source controlled by a controller, where the heating source emits energy for heating the substrate. Also included is a window maintained at a substantially constant temperature. The window is configured to allow a first wavelength spectrum of energy emitted from the heating source to pass through the window. In addition, the window isolates the heating source from an internal region of the processing chamber. A probe is included. The probe is configured to detect a second wavelength spectrum of energy, distinct from the first wavelength spectrum of energy, emitted directly from the substrate. The energy emitted directly from the substrate in the second wavelength spectrum corresponds to a temperature of the substrate, and the temperature of the substrate is provided to the controller, which adjusts an intensity of the heating source based on a set point temperature for the substrate.
In another embodiment, a temperature controlling system for controlling the temperature of a substrate in a chamber is provided. The system includes a heating source and a window. The window is transparent to a first spectrum of wavelengths of energy from the heating source while being opaque to a second spectrum of wavelengths of energy from the heating source. A cooling system is included. The cooling system maintains the window at a substantially constant temperature as the window absorbs the second spectrum of wavelengths of energy from the heating source. A probe is also included. The probe is located remotely from the substrate and configured to detect the second spectrum of wavelengths of energy emitted from the substrate, where the energy emitted from the substrate correlates to a temperature of the substrate. The temperature of the substrate is communicated to a controller of the heating source and the controller of the heating source controls an intensity of the heating source based upon the temperature of the substrate.
In still another embodiment, a method for measuring and maintaining a temperature of a substrate in a processing chamber is provided. The method initiates with providing energy from a heating source. Then, a first wavelength spectrum of the energy from the heating source passes through a window entering an internal region of the processing chamber. The substrate is heated by this first wavelength spectrum of the energy from the heating source. Next, a second wavelength spectrum of energy is filtered prior to entering an internal region of the processing chamber. Then, the energy emitted by the substrate is detected by a non-contact probe. The energy emitted by the substrate has a second wavelength spectrum. Then, an intensity of the heating source is adjusted based upon the detected energy emitted by the substrate.
In yet another embodiment, a method for measuring a temperature of a body in a chamber is provided. The chamber is configured to introduce heat energy through a window. The window is transparent to a first wavelength spectrum of the heat energy and opaque to a second wavelength spectrum of the heat energy. The method initiates with providing a heat source where the heat source emits the heat energy through the window into the chamber. Then, a cooling system for maintaining the window at a substantially constant temperature is provided. Next, the body is heated with the first wavelength spectrum of the heat energy. Then, the temperature of the body is detected, where the detecting includes providing a probe remotely located from the body and the probe configured to detect an intensity of the body""s emissions of the second wavelength spectrum. Also included in the detecting is translating the detected intensity to the temperature of the body.
The advantages of the present invention are numerous. Most notably, contact with the backside of the substrate is not required. In addition, the high intensity of broad-band radiation emanating from the lamps does not affect the probe. The probe is protected from the environment of the chamber and can be adjusted on-line for different substrate backside emissivities. Furthermore, by utilizing high wavelength emissions from the substrate, full advantage is taken of the fact that emissions at high wavelengths are comparatively stronger at the operating temperatures typical of photoresist ashing operations.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.